The relationship between social status, physiological stress and reproductive impairment was investigated in 14 adult female gelada baboons (Theropithecus gelada) living in three semi-free-ranging groups at the Wildlife Conservation Society's Bronx Zoo. Behavioral observations and urine collections were carried out every 1-3 days for 6-18 months. Ovarian function was determined by urinary pregnanediol glucuronide levels, and urinary cortisol levels were used as a physiological marker of stress. Low-ranking females (Ranks 3-8 [N=8]) experienced lower copulation frequencies (p,0.05) and higher rates of aggression and increased urinary cortisol levels (p<0.05) than did high ranking females (Ranks 1 and 2 [N=6]). Receipt of aggression and urinary cortisol concentrations peaked during the follicular phase of the menstrual cycle (p<0.01). The interval between ovulations was greater for low-ranking females than it was for high-ranking females (p<0.05), and the increased length of the menstrual cycle was due to an elongation of the follicular phase (p<0.02), but not of the luteal phase. These results suggest that harassment of low-ranking female gelada baboons by high-ranking females may have induced increased urinary cortisol levels, lengthened menstrual cycles and reduced ovulatory frequency in low-ranking females. High-ranking female gelada baboons may impose stress-induced suppression of reproduction on their female subordinates to obtain a reproductive advantage.